An electronic camera generally converts an optical image into a set of electronic signals. The electronic signals may represent intensities of colors of light received by the camera. The electronic camera typically includes an array of image sensors or light sensitive sensors which detect the intensity of light received by the camera. The image sensors typically generate electronic signals that have amplitudes that are proportionate to the intensity of the light received by the sensors. The electronic signals can be conditioned and sampled to allow image processing.
Integration of the image sensors with signal processing circuitry is becoming more important because integration enables miniaturization and simplification of imaging systems. Integration of image sensors along with analog and digital signal processing circuitry allows electronic camera systems to be low cost, compact and require low power.
Historically, image sensors have predominantly been charged coupled devices (CCDs). CCDs are relatively small and can provide a high-fill factor. However, CCDs are very difficult to integrate with digital and analog circuitry. Further, CCDs dissipate large amounts of power and suffer from image smearing problems.
An alternative to CCD sensors are active pixel sensors. Active pixel sensors can be fabricated using standard CMOS processes. Therefore, active pixel sensors can easily be integrated with digital and analog signal processing circuitry. Further, CMOS circuits dissipate small amounts of power.
FIG. 1 shows a circuit schematic of a prior art active pixel sensor. The active pixel sensor is generally included within an array of active pixel sensors. The active pixel sensor includes a photo-diode D1, a reset transistor Q1, a bias transistor Q2 and a select transistor Q3. The photo-diode D1 collects charge when the photo-diode D1 is exposed to light. The photo-diode D1 includes an inherent capacitance Cd which capacitively loads a signal node N2. The charge collected by the photo-diode D1 is accumulated on the capacitance Cd of the photo-diode D1 creating a photo-diode voltage which is proportional to the intensity of light received by the photo-diode D1. The photo-diode voltage is created at the cathode of the photo-diode D1.
The reset transistor allows the photo-diode D1 to be reset by discharging the photo-diode capacitance Cd. A RST (reset) line discharges the photo-diode capacitance Cd by pulsing the RST line high to set the cathode of the photo-diode to a predetermined reset voltage. The predetermined reset voltage for the active pixel sensor shown in FIG. 1 is voltage potential of the RST line minus the threshold voltage of the reset transistor Q1.
The select transistor Q3 allows a controller to selectively sample the photo-diode voltage at a PIXOUT output of a particular active pixel sensor by pulsing a SELECT line to cause the select transistor Q3 to conduct.
FIG. 2 is a plot of a signal voltage of the photo-diode D1 of the active pixel sensor shown in FIG. 1. The signal voltage is defined as a reference voltage minus the voltage potential of the PIXOUT output. The reference voltage is defined as the voltage potential of the PIXOUT output when the signal node N2 is reset to the predetermined reset voltage. The greater the intensity of light received by the photo-diode D1, the greater the signal voltage. The charge conducted by the photo-diode D1 is proportional to the intensity of light received by the photo-diode D1. As depicted by the plot, the signal voltage begins to saturate as the charge conducted by the photo-diode increases. The saturation voltage V.sub.saturation is the signal voltage in which an increase in the intensity of the light received by the photo-diode D1 does not affect the signal voltage. The saturation of the photo-diode D1 limits the dynamic range of the photo-diode D1. The range of the intensity of light being received by active pixel sensor which is usefully detectable is limited by the fact that the active pixel sensor saturates. Once the signal of the photo-diode D1 of the active pixel saturates, it is impossible to detect changes in the intensity of the light being received by the active pixel sensor. Further, when the intensity of light received by the photo-diode D1 is just below the intensity of light required to saturate the active pixel sensor, the response of the photo-diode D1 is very non-linear. The operation of the active pixel sensor is limited to a range of light intensities in which the response of the photo-diode D1 is linear.
It is desirable to have an active pixel sensor which allows the intensity of detectable light receive by the active pixel sensor to vary over a greater range than presently possible. The active pixel sensor would generate an analog voltage which represents the intensity of light received by the active pixel sensor over a greater range of light intensities than presently possible. Further, the active pixel sensor would be manufacturable using presently existing CMOS fabrication processes.